High Pressure Synthesis And Thermoelectric Properties Of ?Li?Ce?_xCa_yCo₄Sb₁₂ Skutterudites

CoSb₃-based skutterudite compounds are typical medium-temperature thermoelectric materials.The additional charge carrier and vibration of filling atoms in the cage can effectively optimize the electrical transport properties and reduce the lattice thermal conductivity,so as to improve the thermoelectric properties of materials.Elemental filled CoSb₃-based skutterudite compounds,a prototype of phonon-glass electron crystal?PGEC?concept,are among the most promising thermoelectric materials.The traditional preparation methods under atmospheric pressure have some disadvantages,such as long reaction time,the limited of filling elements and filling fraction,and so on.In this thesis,we investigate the high pressure synthesis and thermoelectric properties of single?Ca?and double?Li/Ca,Ce/Ca?elemental filled CoSb₃.The results show that high pressure synthesis can obviously accelerate the reaction process,increase the filling fraction and improve the thermoelectric properties.The filling behavior of Ca element in CoSb₃ under the isoparametric parameters was studied,based on the first principle callculations were performed with CASTEP.According to the filling fraction limit?FFL?theory,the formation energy of the filling phase Ca_yCo₄Sb₁₂ and the second phase of CaSb2.are the determinant factor of the filling fraction limit?FFL?of calcium.The calculation results show that the Ca atoms can be filled into the cages of the CoSb₃ lattice at atmospheric pressure,and the filling fraction is 0.22,which is consistent with the previous results.After introducing pressure parameters,the filling fraction of Ca increases with the increase of pressure.At 1,2,3,4 GPa,the filling fraction of Ca was increased to 0.32?0.40?0.46?0.57,which indicate that pressure can promote the filling of the Ca atom.The Ca-filled CoSb₃ were successfully fabricated with high pressure synthesis?HPS?method and spark plasma sintering at 1 GPa.The produced Ca_yCo₄Sb₁₂ samples were characterized for the structural;all samples are dominated with the skutterudite structure of Im??? symmetry.Compared with the ambient-pressure synthesized samples,the filling fraction of Ca in Co Sb₃ is substantially increased,leading to greatly improved thermoelectric performance of Ca_yCo₄Sb₁₂.The Ca0.31Co4Sb12 smapl shows a maximum power factor 5400 mWm-1K-2,a minmum lattice thermal conductivity 1.1 Wm-1K-1,and a ZT value as high as 1.15 is achieved at 840 K.Finally,we tested the thermoelectric properties of Ca0.31Co4Sb12 samples by rising and falling temperature,and obtained the Ca-filled CoSb₃ samples with good thermal stability.The Li/Ca double-filled CoSb₃ were successfully fabricated with high pressure synthesis?HPS?method and spark plasma sintering at 1 GPa.The synthetic LixCa_yCo₄Sb₁₂ samples possess an Im??? symmetry of skutterudite structure.The thermoelectric properties are greatly improved after Li/Ca double filling.The power factor of Li0.08Ca0.18Co4Sb12 sample maintains a relatively high value?> 4000 ?Wm-1K-2?in the whole measurement temperature range.Combined with the greatly suppressed thermal conductivity?1.33 Wm-1K-1 at 700 K?the highest ZT value of 1.13 is achieved for the Li0.08Ca0.18Co4Sb12 sample.As a result,HPS provides us a fast and effective pathway to fabricate double or multiple filled CoSb₃ samples.The Ce/Ca double-filled CoSb₃ were successfully fabricated with high pressure synthesis?HPS?method at 1 GPa.Structure and thermoelectric properties were investigated.The High pressure synthetic CexCa_yCo₄Sb₁₂ samples possess an Im??? symmetry of skutterudite structure.Compared with Ca or Ce single filled samples,the thermoelectric properties are obviously improved after double filling.The power factor of Ce_0.3Ca_0.3Co4Sb12?nominal composition?sample is more than 5000 ?Wm-1K-2,at temperatures above 700 K,obtain a minmum lattice thermal conductivity 1.69 Wm-1K-1 at 700 K.So a highest ZT value of 1.18 is achieved for the Ce_0.3Ca_0.3Co4Sb12.Our results further confirm the advantages of high pressure synthesis in fabricating double or multiple filled CoSb₃ samples.